In recent years, a valve train mechanism of an internal combustion engine has often adopted a roller type rocker arm including a roller rolling on a cam lobe in order to restrain a loss of power and an abrasion on a contact surface due to a friction with a cam shaft. The roller type rocker arm has hitherto adopted a cast or forged arm body, however, for actualizing reductions in inertial mass of a valve gear system and in manufacturing cost, as disclosed in Japanese Patent Application Laid-Open Nos. 3-172506, No. 7-150909 and so on, a variety of arm bodies have been developed as steel plate press formed products.
The arm body as the steel plate press formed product is manufactured through processes of obtaining a blank by punching out a steel plate, etc in a predetermined contour, thereafter having this blank subjected to plastic deformation by a press die assembly, and forming the arm body constructed, substantially in a U-shape in section, of a pair of side walls and a connection wall connecting these side walls. The arm body is formed with an aperture through which the roller is exposed on the side of the cam shaft, wherein this aperture is formed either by a method of punching out after the press forming of the arm body or by a method of punching out simultaneously when punching the blank out. Note that generally a boss, into which a pivot shaft is screwed, is formed by a burring work when molding the arm body, and through-holes, into and through which the roller shaft is press-fitted and inserted, are formed after having formed the arm body.
Incidentally, in the case of adopting the method of punching out the arm body to form the aperture after the press forming, it is inevitable that an internal peripheral edge of the aperture protrudes more or less as a stepped portion from internal surfaces of the side walls in terms of a problem of a way of setting dimensions of a punch for punching the aperture out. As a result, during an operation of the engine, the roller and a side end surface of a bearing (normally a needle bearing) interferes with the stepped portion of the internal peripheral edge of the aperture. This interference hinders smooth rotations of the roller and might be a factor of causing an abnormal abrasion and an emission of noises. On the other hand, when punched out, burrs are produced on the side end surface of the aperture of a roller holding member and, if bent towards an interior of the arm body, might also interfere with the roller and the side end surface of the bearing thereof.